Help for erring humankind, but why pick one creed? A nun selling religious artifacts in Thessaloniki AFP/Getty Images

Since 2009, an "atheist bus" - one of a fleet being assembled to spread the good word - has been cruising around bearing the legend, "There's probably no God. Now stop worrying and enjoy your life". Francis Spufford writes engagingly: "The implication of the bus slogan is that enjoyment would be your natural state if it weren't for us believers and our hell-fire preaching. Take away the malignant threat of God-talk, and you would revert to continuous pleasure, under cloudless skies. What's so wrong with this, apart from its being total bollocks? "

It is a refreshing response that highlights the most striking feature of contemporary atheism - its invincible incomprehension of actual human beings. For atheist evangelists, practically everything wrong with the world comes from irrationality. In contrast, Spufford argues plausibly that Christianity deals with "the human propensity to fuck things up". HPtFtu - as he abbreviates this central fact of life - denotes "our active inclination to break stuff, 'stuff' here including moods, promises, relationships we care about and our own well-being and other people's."

At bottom HPtFtu is the recognition that human life is not a soluble problem, and people who understand and accept this can cope with situations that in secular terms are truly hopeless. "Virtuous and idealistic atheists are at work all over the place," Spufford writes, "but it is observable that a surprisingly large number of believers are at work with the dying, the demented, the addicted, the institutionalised and the very impaired and afflicted, where the best that can be done is to love for the sake of it" and "keep sorrow company."

HPtFtu is not a uniquely religious insight. The same phenomenon of human self-destructiveness has been recognised by atheists. With his theory of the death instinct Freud is the most obvious example, but another is the neglected Spanish-American philosopher George Santayana (1863-1952), an early mentor of Gore Vidal, who lived and died an atheist while affirming that religion has a vital role in dealing with human frailty. Religious or not, every serious view of things has accepted there is something radically defective about the human animal.

Evangelical atheists believe that HPtFTu can be cured by deciding to be more reasonable – an unrealistic notion, which for me confirms that this sort of atheism is simply too ignorant and childish to be worth bothering with. Rightly, aside from a lengthy footnote in which he dismisses "the steaming heap of 'evolutionary' manure raked together by Richard Dawkins", Spufford does not respond to recent attacks on religion. Much more interestingly, he confronts the failings of Christianity - not least its squalid obsession with sexuality. Unapologetic is a rare gem, a book that carries conviction by being honest all the way through.

That does not mean it is always persuasive. It is one thing to note that religion has an irreplaceable role in dealing with the flaws of the human animal, quite another to suggest that there is anything special about Christianity. Spufford admits that his preference is cultural: "Christianity was the religion of my childhood. It's the ancient religion... of the place I come from." At the same time he thinks Christianity is "right": "It's something I came back to, freely", after 20-odd years of atheism, because "it answers my need, and corresponds to emotional reality for me." Spufford's story is told by a host of people who go back to their ancestral faith. In a society as pluralistic as ours, returning to religion no longer means becoming a Christian.

Since the book is mostly devoted to spelling out the sense of meaning that comes with living as a Christian, it would make sense for Spufford to view religions as ways of life, which may offer intimations of transcendence but cannot make any claims to truth. Instead he endorses the conventional view in which faith is a matter of having the right beliefs. Arguing for the literal truth of Christian narrative, he scoffs at discoveries that have shed new light on the historical Jesus: "a general feeling that somebody... in the early church, probably St Paul, retrospectively glued Godhood onto poor Jesus".

In fact the Jewish identity of Yeshua (Jesus's name before it was Latinised) is far from being just fashionable supposition. In the more literate debates of the 19th and early 20th century it was understood that the real intellectual threat to Christianity would come not from science but history, and this has proven to be the case. Since the Dead Sea Scrolls were discovered, the pre-Christian character of Jesus and his teaching has been established beyond reasonable doubt. The great biblical scholar Geza Vermes has placed Jesus definitively in a tradition of charismatic Judaism. Jesus may have been distinctive, but he was only one of many Galilean holy-men.

Christians tend to ask how it is that the story of Jesus spread when so many other teachers of his day disappeared from history. For anyone who does not already accept a Christian idea of providence, the answer is clear. It was fortuitous events that made Jesus a universal presence. If Paul had not remade Jesus's heterodox Judaism into a proselytising creed, if Constantine had not made Christianity the religion of the Roman empire, if any one of an uncountable number of events had been different, Jesus would have remained the Jewish prophet he was in historical fact.

In that event, the conflict between Christianity and atheism would never have developed. This is the fundamental irony of belief, neglected or denied by Christians and atheists. In matters of religion, what you believe is determined by chance. We would all be better off if we forgot about belief and focused our minds on how we want to live.

'Gray's Anatomy: selected writings' by John Gray is published by Penguin